1. Field of the Invention
The present invention generally relates to an inverter controlled air conditioner and, more particularly, to an inverter controlled air conditioner having an inverter control circuit for effectively reducing a rush current at starting.
2. Description of the Related Art
Some conventional air conditioners have a refrigerating cycle constituted by connecting a capability-variable compressor, a condenser, a pressure reducer, and an evaporator, and control the capability of the compressor in accordance with an air conditioning load in order to obtain an optimum capability corresponding to the air conditioning load, thereby improving the amenity and effect of energy conservation.
An air conditioner of this type has an inverter for driving a compressor motor and a control unit for controlling the inverter. FIG. 5 shows an arrangement of an inverter controlled air conditioner disclosed in Published Unexamined Japanese Patent Application No. 62-126893.
That is, the input terminals of a rectifying circuit 2 having a diode bridge are connected to the phases of a commercial three-phase AC power supply 1 through contactor contacts (normally-open contacts) 21a, and the output terminals of the rectifying circuit 2 are connected to a smoothing capacitor 4 through a parallel circuit of a resistor 3 and a relay contact 22a. A switching circuit 5 is connected to the two terminals of the smoothing capacitor 4, and the output terminals of the switching circuit 5 are connected to a compressor motor 6 for driving a compressor in an air conditioner body 8.
The resistor 3 serves to prevent a rush current from flowing in the smoothing capacitor 4 when the contactor contact 21a is closed to supply power. The relay contact 22a serves to form a short circuit for the resistor 3 when it is assured that no rush current flows.
The rectifying circuit 2, the smoothing capacitor 4, and the switching circuit 5 constitute an inverter 7.
The three-phase AC power supply 1 is also connected to a control section 10. The control section 10 is constituted by an auxiliary power supply circuit 15, an inverter control circuit 16, and relay drive circuits 17a and 17b. The auxiliary power supply circuit 15 comprises a rectifying circuit 11 for rectifying an AC voltage (line voltage) of the three-phase AC power supply 1, a smoothing capacitor 13 connected to an output terminal of the rectifying circuit 11 through a resistor 12, and a switching transformer (DC-DC converter) 14 connected to the two terminals of the capacitor 13. The inverter control circuit 16 comprises a CPU and peripheral circuits and obtains an operation voltage of 5V from the DC-DC converter 14 described above to control the inverter 7.
The relay drive circuits 17a and 17b of the control section 10 serve to selectively supply an operation voltage of 12V from the DC-DC converter 14 to an electromagnetic contactor 21 and a DC relay 22.
The function of the inverter controlled air conditioner shown in FIG. 5 will be described with reference to FIG. 6.
When a start command is input to the air conditioner body 8 as the drive target (YES in step S1), the electromagnetic contactor 21 is energized by the relay drive circuit 17a through the CPU in the inverter controlled circuit 16 (step S2), and the internal timer of the CPU starts counting t (step S3).
When the electromagnetic contactor 21 is energized, its contacts 21a are closed, and the voltage of the power supply 1 is applied to the inverter 7.
At this time, although a rush current is to flow to a main circuit starting from the rectifying circuit 2 and ended with the smoothing capacitor 4, it is decreased by the resistor 3. As a result, the smoothing capacitor 4 can be protected from a damage by the rush current at the starting.
When the timer count t of the internal timer of the CPU of the inverter control circuit 16 reaches a predetermined time t.sub.s (YES in step S4), the relay 22 is energized by the relay drive circuit 17b (step S5).
When the relay 22 is energized, the contact 22a is closed to short-circuit the resistor 3. Hence, a normal main circuit is formed, and the standby mode capable of driving the switching circuit 5 any time is set.
While no stop command is input to the air conditioner 8 (NO in step S6), if the operation ON state of the air conditioner body 8 is continued (YES in step S7), the switching circuit 5 is driven (step S8). More specifically, a voltage having a predetermined frequency is output from the switching circuit 5 to drive the compressor motor 6. Then, the air conditioner body 8 starts operation.
During the operation, if a stop command is input to the air conditioner body 8 (YES in step S9) or if an operation OFF command is output from the air conditioner body 8 (YES in step S6), the electromagnetic contactor 21 and the relay 22 are deenergized (steps S10 and S11). This sets the air conditioner body 8 in the stop mode.
In the inverter controlled air conditioner described above, a large current of about 30 A flows in the main circuit of the inverter 7 at starting. For this reason, the relay 22 and the resistor 3 both for preventing damages by the rush current need to have a large capacity, leading to an increase in cost.
In practice, since the relay 22 for preventing a rush current need to operate only when the inverter controlled air conditioner is started, it is operated only several times a day at maximum. Therefore, it is not preferable to employ a large-capacity, expensive relay as a relay which is not used very often in terms of cost performance.